I seem to remember reading somewhere (maybe the Wall of Sound Road Trips?) that Phil said playing in front of the wall was like being under water because the sound was so thick for lack of a better description.
Ha!! You are correct it was the booklet in the RT release!!
"it was simply too loud on stage. The theory had been that the music would go over their heads, but instead, said Lesh the top layers acted as a ceiling. "It felt like I was underwater with pressure most of the time.""